Add TOCI_OVERCLOUD_ALL_IN_ONE option.

Adds a new option to support testing with an all-in-one
overcloud template. When the option is used only a single
overcloud image gets built which contains everything.

This will require a new heat template which is added here:

  https://review.openstack.org/56468

Change-Id: I455fd03e9181dcf5f4ea683d1b2fb8435002fb89